The condenser microphone is a widely used type of microphone. In some respects, this microphone can be considered a variable capacitor whose capacitive value is modulated by the pressure of an incoming sound wave. In this view, one of the capacitor plates is static, while the other one is mobile (i.e., the moving diaphragm component). The sound wave changes the distance between the plates, and this respectively changes the capacitance of the representative capacitor.
The MEMS microphone is in some aspects a variant of a condenser microphone and is produced by using silicon micro-fabrication techniques. Compared to the conventional microphone, the MEMS microphone has several advantages such as a reduced size, lower temperature coefficient and higher immunity to mechanical shocks. In addition, the MEMS microphone takes advantage of lithography processes, which are particularly suitable and advantageous for the mass production of devices.
One approach to obtain a useful electrical signal from such microphone is to maintain a constant charge Q on the capacitor. The voltage across the capacitor will change inversely proportionally to the incoming sound wave pressure according to the equation V=Q/C, consequently dV=−VdC/C. In practice dC/C is relatively small because of mechanical and linearity considerations. In order to get sufficient sensitivity, a high DC voltage V across the capacitor is needed.
Metal Oxide Semiconductor (MOS) devices are quite sensitive to Electro-Static Discharge (ESD) damage. The problem is especially pronounced in deep submicron Complementary Metal Oxide Semiconductor (CMOS) processes because the gate oxide of the transistors is just few nano-meters thick. In order to protect chips, inputs and outputs are often equipped with dedicated ESD protection circuitry. Unfortunately, previous attempts at solving the ESD discharge problem using this circuitry have shortcomings and have failed to adequately address the problem.
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